Re: [Qemu-devel] [PATCH 1.1] coroutine: Avoid ucontext usage on i386 Linux host

[Anthony Liguori <aliguori@us.ibm.com>]

On 05/09/2012 02:34 PM, Jan Kiszka wrote:
> On 2012-05-09 16:27, Michael Tokarev wrote:
>> On 09.05.2012 23:21, Jan Kiszka wrote:
>>> On i386, glibc only saves/restores the signal mask via sigprocmask,
>>> excluding RT signal. A Linux bug in the compat version of this syscall
>>> corrupts the RT signal state, which will cause lockups of QEMU's VCPU
>>> threads.
>>
>> This should obviously be fixed in kernel, for benefit of all (not only
>> qemu), do you have any details here?
>
> compat_sys_sigprocmask reads 32-bit sigmask from user space, i.e.
> excluding RT signal, but calls sys_sigprocmask that takes a 64-bit
> sigset. So the RT signals are unblocked. I'm testing a simple patch ATM,
> will post it to LKML once this works.
>
>>
>>> Signed-off-by: Jan Kiszka<jan.kiszka@siemens.com>
>>> ---
>>>
>>> I'm not sure where to fall back to. The existing code uses gthread,
>>> likely because it is the safer harbor. So I picked it as well.
>>
>> Can't we resort to the SIGUSR1 workaround for the time being, while
>> no RT signals are in actual use, and just have the time to let the
>> kernel side to fix the things up before some actual RTsig user will
>> emerge in qemu?  I think it is a bit more conservative approach,
>> especially having in mind the minority of users this issue affects
>> (only 32/64 mixed environment).  I'd favor for this variant, and
>> it looks like I'm the "main" 32/64bit user of qemu in this world :)
>
> Most conservative is definitely this patch, not switching to SIGUSR1,
> hoping that no other RT signal user shows up until current kernel are no
> longer in use.

Sorry, how is using a totally different code path more conservative than using a 
different signal number?

Why would we even use an RT signal in the future?

Regards,

Anthony Liguori

>
> Jan
>